The first thing that comes to mind is my siphon starter (the racking cane inside of a wine thief thingy). At first I thought that it was gimmicky, but I love how easy it makes transfers. Not to mention sneaking samples of old mead.Get one, it's worth every penny of the ~$12 it cost

Oh, and no homebrewery should be without a spray bottle full of star san. That's probably about as essential as it gets.

refractometer... no more waiting until just before I pitch the yeast to see how I did.

my other is 4 gallon plastic water bottles. I use them the store grain, others to hold brew water for a day or two to disapate chlorine, I use one for my gravity feed recirculate wort chiller*

* I don't have a good connection indoors for my wort chiller. I fill a bottling bucket with water and ice and gravity feed to the kettle that is on the floor. The output goes to a water bottle. The first 4 gallons of very hot water is saved for cleanup. From then on I recirculate (lift the output water bottle and refill the bottling bucket) until I get down to pitching temp.